1. Technical Field
This invention relates to the field of electromagnetic radiation sources. More particularly, this invention relates to point or line radiation sources with an arbitrarily variable spectrum and systems and methods employing same.
1. State of the Art
All objects of non-zero absolute temperature emit thermal radiation. Spectral energy density f(xcex,T) is given by Planck""s radiation law:                                           f            ⁡                          (                              λ                ,                T                            )                                =                                    8              ⁢              π              ⁢                              xe2x80x83                            ⁢              hc              ⁢                              xe2x80x83                            ⁢                              λ                                  -                  5                                                                    (                                                e                                                            hc                      /                      λ                                        ⁢                                          xe2x80x83                                        ⁢                    kT                                                  -                1                            )                                      ,                            (        1        )            
which is strictly valid for a blackbody, where h=6.626xc3x9710xe2x88x9234 J S (Planck""s constant), c=2.998xc3x97108 m/s (speed of light), xcex is wavelength, k=1.381xc3x9710xe2x88x9223 J/K (Boltzman constant), and T is absolute temperature. For bodies at room temperature (T=300 K), this yields a spectrum with a maximum intensity at approximately 10 xcexcm wavelength in the middle infrared spectral range. If the temperature is increased, the spectral energy distribution will vary according to equation (1), and the wavelength at maximum intensity (xcexmax) will be displaced towards shorter wavelengths. For T=6000 K, the temperature of the surface of the sun, xcexmax is in the visible range. This displacement of xcexmax as a function of temperature is approximated by Wein""s displacement law:
xe2x80x83xcexmaxxc2x7T=constant=2.898 33 10xe2x88x923mxc2x7s,xe2x80x83xe2x80x83(2)
which can be derived from equation (1). By integration over all radiation frequencies, one derives Stefan-Boltzmann""s radiation law:
R="sgr"T4,xe2x80x83xe2x80x83(3)
where the total emittance, R, is the total energy of all wavelengths emitted per unit time and per unit area of the blackbody, T is the kelvin temperature, and "sgr" is the Stefan-Boltzmann constant, equal to 5.672xc3x9710xe2x88x928 W/m2K4. It should be noted that the total emittance for an outside surface of an object is always somewhat less than R in equation (3), and is different for different materials. A good approximation of total emittance for non-blackbody objects is:
R=xcex5"sgr"T4,xe2x80x83xe2x80x83(4)
where xcex5 less than 1, and is termed the object""s emissivity.
Electromagnetic radiation sources are used in products ranging from lights to X-ray machines. For example in a conventional infrared spectrometer, one will typically find a hot radiation source, an optical filter that selects a restricted spectral region from the continuum of radiation emitted by the source, a chamber containing a sample which is radiated, and a detector that measures radiation passed through the sample. Usually, the radiation sources of such spectrometers operate at a constant temperature Th, which is much higher than the background, or ambient, temperature, T0.
For many practical instruments it is useful to modulate the emitted radiation either spectrally, temporally, or both. One conventional method of creating pulsed radiation is to insert a rotating wheel (a chopper) furnished with equidistant apertures along the rim, into the radiation path to make the radiation pulsed. Pulsed radiation is particularly useful because many types of infrared detectors only respond to changes in radiation level. For example, pyroelectric detectors used in applications of photoacoustic spectroscopy and related techniques, require pulsed radiation. Pulsed radiation is also advantageous in electronic amplification and noise discrimination.
A non-mechanical means of providing pulsed radiation is disclosed by Nordal et al. in U.S. Pat. No. 4,620,104. In Nordal et al., thick film resistors mounted on ceramic substrates are electrically heated with pulsed current to generate pulsed infrared radiation without the use of mechanically moving parts. However, Nordal et al. appears to be limited to producing infrared radiation because the resistors can only be heated to limited temperatures, i.e., T less than 800 K.
The Nordal et al. reference (U.S. Pat. No. 4,620,104) also discloses a spectrometer based on an infrared radiation source, but it is limited to the infrared spectral range because of the light source employed.
Thus, there is a need in the art for spectrally encoded radiation sources, not limited to infrared spectral radiation, and systems based on same.
The present invention includes an apparatus and method for producing a spectrally variable radiation source and systems including same. An embodiment of a spectrally variable radiation source is disclosed including: a broadband radiation source array, a collimating element, a dispersive element, an imaging element, an output aperture and an optional output collimating element.
An embodiment of a spectrally encoded infrared chromatograph incorporating a spectrally variable radiation source is disclosed. The embodiment of a spectrally encoded infrared chromatograph includes an infrared emitter array, a collimating element, a dispersive element, an imaging element, an output aperture, an output collimating element, a beam splitter, a measurement beam focusing element, a sample cell, a measurement signal detector, a reference focusing element, a reference signal detector, and a processor for processing and displaying sample spectra.
An arbitrary spectrum projector for simulating emission or absorption spectra for chemical and biological agents, as well as projecting calibration and test spectra for characterizing sensors, is also disclosed. An embodiment of a arbitrary spectrum projector includes an emitter array, a transmitting lens, a diffraction grating, a collimating lens, a focusing lens, an adjustable slit with a reflective back, a blackbody radiator, a mirror for reflecting blackbody radiation, and an output collimating lens.